For a semiconductor manufacturing process, a pressure control device is used for introducing a fluid such as a material gas at a predetermined constant pressure into a chamber that is maintained in vacuum.
More specifically, the pressure control device is arranged in the upstream side of the chamber in the flow channel, and comprises a pressure sensor, a pressure control valve and a valve control part that controls the pressure control valve so as to make a measured pressure value measured by the pressure sensor at a set pressure value.
Conventionally, only an accuracy of the pressure control in a steady state at a time when a predetermined period of time passes from an initial state is regarded as important for the pressure control of the fluid by the pressure control device, however, recently an accuracy of the pressure control or a responsiveness in a transition state which is a time period between an initial state and a steady state is also regarded as important.
For example, in a semiconductor manufacturing process, it is required to further improve the accuracy or the responsiveness of the pressure control in the transition state while a response time which is a time period from the initial state to the steady state is shortened as much as possible. More concretely, if the set pressure value is large, the flow rate of the fluid required to be filled into the chamber also becomes large. As this result, there is a problem that the responsiveness is decreased because the response time becomes longer compared with a case that the set pressure value is small.
One method to solve the problem represented improves the responsiveness of the pressure control by setting a large value as the PID coefficient (gain) in a case where the valve control part controls the open degree operated amount of the pressure control valve by means of the PID control.
However, if a large value is simply set as the gain, such as the PID coefficient, in a case where the set pressure value is large, it is possible to shorten the response time because a large value of the open degree operated amount is input. However, in a case where the set pressure value is a target value that is small, the pressure control might be unstable because the pressure control valve is controlled at an excessive open degree operated amount although it is unnecessary to fill the fluid into the chamber so much. In other words, if the PID coefficient is determined to be large based on the large set pressure value as a reference, in a case where the set pressure value is small, the stability of the control is lost so that ringing might be generated or a big overshoot might be generated in the transition state as shown in FIG. 10. In order to make it possible to realize both the good responsiveness and the good stability at all of the set pressure values, the PID coefficient is changed according to the set pressure value. However, with this arrangement, since it is necessary to experimentally determine the PID coefficient for each of the set pressure values, it becomes very troublesome to adjust the parameter.
Meanwhile, Patent Document 1 discloses a pressure control device wherein an upper limit value (a limit) is provided for the open degree operated amount of the pressure control valve in the transition state without changing the PID coefficient according to the set pressure value so as not to produce overshoot and to obtain the preferable responsiveness.
This pressure control device comprises a PID control part that conducts a PID calculation on the deviation between the set pressure value and the measured pressure value and that outputs the open degree operated amount of the pressure control valve and a limiter part that sets an upper limit value to the open degree operated amount output by the PID control part during a transition response period from an initial state, which is a time of initiation of raising the pressure to a steady state.
The limiter part is so configured that the upper limit value of the open degree operated amount is increased in proportion to the elapsed time from the time of the initiation of raising the pressure to the steady state. More specifically, in a case where the pressure control valve is continuously controlled at the open degree operated amount of the upper limit value from the time of the initiation of raising the pressure, the open degree gradually increases in accordance with the elapsed time.
However, with this arrangement, since the upper limit value of the open degree operated amount of the pressure control valve is fixed regardless of the elapsed time from the time of the initiation of raising the pressure, the time period to reach the steady state changes largely depending on the set pressure value. Concretely, even though the set pressure value is set at a large value such as 100% and it is required to raise the pressure to the set pressure value in a short period of time, since the limiter part sets the upper limit value of the open degree operated amount at a small value at a time of initiation of raising the pressure, the fluid does not flow into the downstream side so much so that the rising amount of the pressure also becomes small. As a result of this, in a case where the set pressure value is large, the response time to reach the steady state becomes longer as compared with a case in which the set pressure value is set at 50% or the like.
On the other hand, as shown in the patent document 1, if the upper limit value of the open degree operated amount is changed according to the elapsed time, the response time from the initial state to the steady state varies depending on the set pressure value so that it is not possible to maintain the response time. As a result, in the semiconductor manufacturing process, the longest time period until reaching the steady state is set as a waiting period so as to make it possible to introduce the material gas stably into the chamber, however, especially in a case where the set pressure value is small, an unnecessary waiting time is generated so that a yield is lowered.
In addition, the above-mentioned problems are also generated for a flow rate control device on a pressure basis that conducts a flow rate control based on a measured flow rate value output by the flow rate sensor of pressure type. More specifically, in a case of the flow rate sensor of pressure type, the flow rate is measured based on the pressure in the upstream side and the pressure in the downstream side of the fluid resistance where the pressure loss is generated, if the chamber is connected in the downstream side, the pressure in the downstream side is maintained generally in vacuum or at a constant value so that it becomes substantially equal to control the pressure value of the fluid in the upstream side of the fluid resistance. Accordingly, if the PID coefficient is set to be large in order to shorten the response time even though the set flow rate is large similar to the above-mentioned pressure control device, a problem of generating overshoot or ringing might be generated because the stability is lost in a case where the set flow rate is small.